Rechargeable Li ion batteries have a wide range of applications. They are used to supply electricity to many portable electronic devices and hand-held tools, such as laptop computers, cell phones and other wireless communication devices, cordless electrical tools, and others. They can also be used in automobiles, trucks, airplanes, and other mobile devices either as the primary or sole power source, or as an auxiliary power source. An example is their use in hybrid vehicles and electric vehicles. Rechargeable batteries also can be used as a device to store electricity generated from intermittent sources, such as wind turbines or solar panels.
The performance of the Li ion battery is important in any of these applications. In general, the performance is measured by the charge density or storage capacity (how much electric charge can be stored per unit weight or volume), power density (rate of discharge per unit weight or volume), cycling durability (the number of charge-discharge cycles that can be repeated while maintaining the storage capacity and power delivery capability), and safety. The first three, namely storage capacity, power density, and cycling durability are determined primarily by the electrically active components of the battery.
The storage capacity, power density, and cycling stability depend strongly on the nature of the electrically active material (EA) and how it is supported and electrically connected to the current collector, which transfers electrons between the EA and the outside world. In a typical commercial Li ion battery, the negative electrode uses graphite powder as the EA, which is bonded together and to a metallic current collector with a binder. The maximum storage capacity of the graphite is determined by the chemical stoichiometry as one Li per six carbon atoms, giving a charge density of about 380 mAh/g of graphite. The storage capacity can be increased significantly using other EAs, such as Si, Sn, and many other elements as well as bimetallic or multimetallic mixtures. A major obstacle to the use of these alternative EAs is cycling stability. For example, the theoretical storage capacity of Si is about 10 times higher than graphite, but for negative electrodes made of silicon nanoparticles (e.g., particles of tens of nm diameter), the initial high capacity is lost after a few cycles to less than 10% of the theoretical capacity.
Silicon is often used as an example of an electrically active material, being an attractive candidate because it possesses the highest theoretical energy density among common elements, is cheap, and easy to handle. Various forms of Si electrode materials have been tested, including Si particles mixed with a binder and conducting carbon, nanowires, thin films, and 3-dimensional porous particles. (See, for example, B. A. Boukamp, G. C. Lesh and R. A. Huggins, J. Electrochem. Soc., 1981, 128, 725-729; B. Gao, S. Sinha, L. Fleming and O. Zhou, Advanced Materials, 2001, 13, 816-819; J.-K. Lee, M. C. Kung, L. Trahey, M. N. Missaghi and H. H. Kung, Chem. Mater., 2009, 21, 6-8; C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol., 2008, 3, 31-35; T. Takamura, M. Uehara, J. Suzuki, K. Sekine and K. Tamura, J. Power Sources, 2006, 158, 1401-1404; and H. Kim, B. Han, J. Choo and J. Cho, Angew. Chem., Int. Ed., 2008, 47, 10151-10154, S10151/10151-S10151/10153). However, these are still not satisfactory, either because of poor cycling stability, cost of manufacturing, and/or insufficient capacity improvement. Although the exact causes for storage capacity loss upon cycling are still under investigation, one contribution is fracturing of the Si structure consequent to the large volume changes upon lithiation/delithiation, resulting in loss of electrical contact of some Si fragments. Various attempts to stabilize these structures have been reported. The most common approach is to encapsulate the Si structure with a conducting carbonaceous layer, in hope that this would better retain the Si fragments from being disconnected from the conducting electrode. Various precursors can be used for encapsulation, including resorcinol-formaldehyde gel, poly(vinyl chloride)-co-vinyl acetate or polyvinyl chloride and chlorinated polyethylene, glucose, and fullerene C60. (See, for example, J. K. Lee, M. C. Kung, L. Trahey, M. N. Missaghi and H. H. Kung, Chem. Mater., 2009, 21, 6-8; Y. Liu, Z. Y. Wen, X. Y. Wang, X. L. Yang, A. Hirano, N. Imanishi and Y. Takeda, J. Power Sources, 2009, 189, 480-484; Q. Si, K. Hanai, N. Imanishi, M. Kubo, A. Hirano, Y. Takeda and O. Yamamoto, J. Power Sources, 2009, 189, 761-765; Y. S. Hu, R. Demir-Cakan, M. M. Titirici, J. O. Muller, R. Schlogl, M. Antonietti and J. Maier, Angewandte Chemie-International Edition, 2008, 47, 1645-1649; and A. A. Arie, J. O. Song and J. K. Lee, Mater. Chem. Phys., 2009, 113, 249-254). Noticeable improvements were achieved, but capacity degradation was not eliminated.
In many of the engineered structures examined, such as nanowires and thin films, Si and other high capacity materials exhibit storage capacities that are near the theoretical value. However, the need to maintain electric conductivity with the current collector limits their dimensions to hundreds of nanometers. Furthermore, these structures typically require a metallic current collector as support, the weight of which significantly lowers the overall electrode storage capacity of the electrode assembly.